A 42‑year‑old woman presented with ~30 adenomatous polyps of the left sided‑colon with early rectosigmoid cancer. The patient had no previous medical history and no familial history of inherited colorectal disease. No germline gene mutations associated with colorectal adenomatous polyposis, including APC regulator of WNT signaling pathway, mutY DNA glycosylase, DNA polymerase‑ε, catalytic subunit, DNA polymerase δ1, catalytic subunit, and mismatch repair genes, were detected via germline genetic testing. A heterozygous germline mutation in methyl‑CpG binding domain 4, DNA glycosylase (MBD4), c.217C>T/p.Gln73*, which resulted in the generation of a stop codon, was identified by genetic analyses including whole‑exome sequencing. Immunohistochemical staining analysis revealed that the expression of MBD4 protein was absent in the cancer tissue, while it was expressed in the normal epithelium. Sequencing and copy‑number analyses demonstrated the loss of the remaining allele of MBD4 in the cancer tissue. Furthermore, somatic mutation signature analysis showed preferential transition of cytosine to thymine residues at CpG dinucleotides in cancer tissues. Although it has been previously reported that germline missense mutations and somatic mutations of MBD4 are associated with the development of colorectal cancer, this is the first report, to the best of our knowledge, in which a germline nonsense mutation of the MBD4 gene has been identified in an early‑onset colorectal cancer patient with oligopolyposis.